In known apparatus of this type, the cold air may consist either of fresh air drawn from outside the cabin, which may then be air Conditioned, or else recycled air drawn from inside the cabin.
The mixed air is air which has been adjusted in temperature by mixing two streams of air together in adjustable proportions, namely a stream of the said cold air and a stream of warm air. The warm air is obtained by heating all or some of the cold air in a heat exchanger, typically by heat transfer from the coolant liquid of the engine of the vehicle. The temperature of the mixed air is thus always greater than that of the cold air. The mixed air is then delivered through various air vents, so as to be distributed into different parts of the cabin of the vehicle in accordance with selected distribution modes. These air vents generally comprise at least one deicing and de-misting vent arranged at the base of the windshield, at least one vent communicating with the lower part of the cabin for heating the feet of the occupants, side vents arranged at the two ends of the fascia, and at least one further vent which is arranged on the fascia so as to direct air towards the bodies and faces of the front occupants of the vehicle.
This last mentioned vent, which is typically arranged in the middle of the fascia and which may also be referred to as an aerator, or an aerator vent, delivers not only mixed air but also cold air, so that the occupants of the vehicle receive air at a lower temperature than that of the mixed air which is delivered behind the windshield or towards the feet of the occupants or the side windows. In this way the air is delivered at two different temperature levels, which leads to improved comfort and improved safety for the occupants, and particularly for the driver.
One apparatus of the type discussed above is known from the specification of German published patent application No. DE 3 338 768A, in which a single flap valve is provided for controlling the cold air stream and the mixed air stream at the outlet of the cold and mixed air ducts and at the inlet of a common duct which terminates at the aerator vent. The main drawback of that arrangement lies in the fact that it is not possible to modify the flow of cold air and mixed air independently of each other. As a result, this known apparatus either delivers cold air and mixed air simultaneously, though always in the same proportion, or else it interrupts the delivery of both the cold air and the mixed air.
Due to the fact that the cold air and the mixed air are always delivered in the same proportion through the common aerator vent, the temperature of the air leaving this vent is always the same at any given setting of the temperature of the mixed air.